The present invention relates to a high-pressure metal vapor discharge lamp having a discharge tube made of transparent ceramics.
Conventionally, high-pressure metal vapor discharge lamps of this type often are known to have a quartz discharge tube as shown in FIG. 9. In other words, a quartz discharge tube 34 having a pair of main electrodes 31 and 32 and an auxiliary electrode 33 therein is provided, and the auxiliary electrode 33 is disposed next to the main electrode 32. The discharge tube 34 also has a discharge portion 35 as a discharge space and sealing portions 36 and 37 that are provided at both ends of the discharge portion 35. In the sealing portions 36 and 37, electrode rods 38 and 39 supporting the main electrodes 31 and 32 at their tips, metal foils 40 and 41 that are made of molybdenum and whose one end is connected to the rear end of the electrode rods 38 and 39 and external lead wires 42 and 43 whose one end is connected to the other end of the metal foils 40 and 41 are integrated to be current supply conductors for the main electrode, and an auxiliary electrode rod 44 supporting the auxiliary electrode 33 at the tip, a metal foil 45 that is made of molybdenum and whose one end is connected to the rear end of the auxiliary electrode rod 44 and an external lead wire 46 whose one end is connected to the other end of the metal foil 45 are integrated to be a current supply conductor for the auxiliary electrode. The current supply conductors for the main electrode and the current supply conductor for the auxiliary electrode are sealed by crushing so that the main electrodes 31 and 32 and the auxiliary electrode 33 at their tips are located in the discharge portion 35. Also, a Nexe2x80x94N2 mixed gas is sealed in an outer tube 2.
When starting the operation of such a high-pressure metal vapor discharge lamp, an auxiliary discharge first is generated between the main electrode 32 and the auxiliary electrode 33 provided next to this main electrode 32, followed by a transition to a main discharge between the main electrodes 31 and 32.
In particular, metal halide lamps, which are one type of the high-pressure metal vapor discharge lamps having the above-described structure, are used widely as ones to which inexpensive ballast for mercury lamps are applicable.
JP 62(1987)-150646 A, directed to a ceramic discharge lamp, discloses a ceramic discharge tube with the following structure. That is, as shown in FIG. 10, electrically conductive cermet disks 53a and 53b supporting main electrode rods 52a and 52b are sealed air-tight at the ends of a discharge tube 51. The disk 53a supports an auxiliary electrode 54 so that the auxiliary electrode 54 is insulated from the main electrode rod 52a via an insulating layer 55.
Also, JP 10(1998)-106491 A, directed to a high-pressure metal vapor discharge lamp, discloses the following structure. That is, a discharge tube 61 is disclosed to have a structure in which, as shown in FIG. 11, transparent ceramic disks 64a and 64b provided with slender ceramic tubes 63a and 63b in which electrode lead-in wires 62a and 62b as electrode lead-in members are sealed are provided at both ends of a main tube 68 made of transparent ceramics, and that the disk 64a further is provided with a slender ceramic tube 63c for the auxiliary electrode.
However, those types of discharge lamps have had following problems.
In the high-pressure metal vapor discharge lamp including the conventional quartz discharge tube as shown in FIG. 9, the sealing portions of the discharge tube are sealed by crushing during its manufacture, resulting in varying shapes. This shape variation has caused variation in lamp characteristics.
When the sealing portions are large, thermal loss from a discharge space of the discharge tube increases, making it difficult to obtain a sufficient efficiency and a high color rendition. Thus, it is necessary to reduce the size of the sealing portions as much as possible. However, in the quartz discharge tube, since it is necessary to seal the metal foil 41 on the main electrode side and the metal foil 45 on the auxiliary electrode side so as to space them away at a certain distance and prevent their contact, the size reduction of the sealing portion 37 has been difficult.
Also, in this type of high-pressure metal vapor discharge lamp, a Nexe2x80x94Ar mixed gas is used as a starting gas for the purpose of lowering a starting voltage at discharge start-up. However, since Ne permeates the quartz as a discharge tube material, a mixed gas containing Ne needs to be sealed in the outer tube 2 so as to prevent this permeation. On the other hand, when the gas is sealed in the outer tube 2, the thermal loss from the discharge tube 34 increases. Accordingly, in order to obtain sufficient characteristics, it is necessary to tolerate a deterioration of lifetime characteristics to a certain degree and increase a tube-wall load of the lamp. Such deterioration of lifetime characteristics is caused by a reaction between quartz in a wall portion of the discharge tube and a metal halide that has been sealed. Thus, it has been desired that such reaction between the quartz as the discharge tube material and the sealed metal halide is suppressed.
On the other hand, in the lamp disclosed in JP 62-150646 A using ceramics for the discharge tube, variation in the shape of the discharge tube can be suppressed, thus making it possible to improve quality regardless of the presence or absence of the gas in the outer tube. However, since an electrically conductive cermet 56 having the auxiliary electrode 54 is sealed air-tight with a sealing material 57 in a portion reaching a relatively high temperature during the lamp operation, leaks in the discharge tube during the operation or the reaction between the sealing material and enclosed metals are inevitable. Especially, in metal halide lamps using metal halide as a discharge metal, a violent reaction occurs.
In the lamp disclosed in JP 10-106491 A in which the electrode lead-in members are sealed in the slender ceramic tubes, the reaction between the sealing material and the enclosed metals can be avoided. However, when attempting to obtain a reliable mechanical strength of the disk 64a, it is difficult to reduce the distance between a main electrode 65a and an auxiliary electrode 66 and the shape of end portions of the discharge tube becomes limited. Consequently, it is difficult to design the discharge tube for the purpose of obtaining desired lamp characteristics.
The present invention was made in order to solve the problems described above, and it is an object of the present invention to provide a high-pressure metal vapor discharge lamp that prevents the occurrence of variations in characteristics caused by shape variation of a discharge tube as in a quartz discharge tube, achieves highly efficient and stable lifetime characteristics independent of the presence or absence and the composition of a gas in an outer tube, suppresses leaks during a lamp operation and characteristic changes caused by a reaction between a sealing material and an enclosed material, has stable lamp starting characteristics, and allows a free design of the discharge tube.
A high-pressure metal vapor discharge lamp of the present invention includes an outer tube sealed air-tight by a stem, and a discharge tube of transparent ceramic in which mercury, a rare gas and a discharge metal are sealed, the discharge tube being inside the outer tube. The discharge tube includes a main tube, a pair of slender tubes disposed at both ends of the main tube, at least a pair of main electrodes located in the main tube and at least an auxiliary electrode located in the main tube. The pair of main electrodes are connected to electrode lead-in members that are sealed in the slender tubes with a sealing material, the auxiliary electrode is connected to an auxiliary electrode lead-in member, and the auxiliary electrode lead-in member connected to the auxiliary electrode is isolated electrically from the electrode lead-in member connected to the main electrode and sealed in the slender tube with a sealing material.
Unlike a conventional high-pressure metal vapor discharge lamp using the quartz discharge tube, this structure can eliminate the shape variation of the discharge tube, which has been inevitable conventionally. Thus, it is possible to reduce the variations in lamp characteristics caused by this shape variation. Also, the reaction between the enclosed metals and the discharge tube can be suppressed, thus reducing variation in optical characteristics of the lamp, making it possible to reduce the characteristic change during its lifetime.
Also, it becomes possible to lower the temperature of the sealing material during the operation, thus preventing corrosion of the sealing material by a reaction between the enclosed metals and the sealing material easily. Accordingly, reliability can be improved compared with the conventional structure of the sealing portions.
Furthermore, since the auxiliary electrode lead-in member to which the auxiliary electrode is connected and the electrode lead-in member to which the main electrode is connected are sealed in the same slender tube, it becomes possible to reduce the distance between the auxiliary electrode and the main electrode next to it, thus lowering a starting voltage. Moreover, since it is sufficient that one slender tube each is disposed at both ends of the discharge tube, the discharge tube can be designed in a relatively free manner.
In addition, an alternative main electrode can be used instead of the auxiliary electrode (or in addition to the auxiliary electrode) in the above-described structure of the present invention. This achieves the structure in which a switching element selects the main electrode or the alternative main electrode to light the lamp.
Such structure reduces the frequency at which the main electrode or the alternative main electrode is exposed to sputter caused by a high voltage at lamp start-up or to high temperature during the lamp operation, thereby suppressing the consumption of each electrode.